High Oil Pressure

[Dave62]

Bob,
Believe it....the only way to change flow in a fixed orifice system is to either change media....or change the length of the plumbing ...this will have a significant impact due to the resistance to flow. Another thing that will affect flow is the number and sharpness of bends. This acts like a resistor due to turblance at the intersection. I know that is not logical but thats the physics of a system. What will change as you change pressure is the distance the fluid will squirt out of the end of the pipe.
Dave

[Mel]

Originally Posted by Dave62

What will change as you change pressure is the distance the fluid will squirt out of the end of the pipe.
Dave

Definitely! I have over 95 psi of water pressure at my house. My sprinklers do cover a good distance.

[skylor]

Originally Posted by Dave62

Bob,
Believe it....the only way to change flow in a fixed orifice system is to either change media....or change the length of the plumbing ... What will change as you change pressure is the distance the fluid will squirt out of the end of the pipe.
Dave

This thread is drifting a bit off course but:

Fixed orifice flow rate is a function of upstream pressure (and down stream pressure for non-cavitating flow). If Bob opens his bathtub faucet half-way, then reduces the supply pressure at the faucet (I don't care how...reducing the pressure regulator setting, opening another faucet in the house, flushing a toilet, etc), he will reduce the volumetric flow rate through the bathtub faucet.

Skylor (ME with lots of fluid system experience)

[airguy]

Originally Posted by rocketbob

ok. Lets say I leave the faucet half open, and I open the kitchen faucet half way as well. Therefore both are in a constant state of leakage. Then I go down in the crawl space and start playing around with the pressure relief valve. You're telling me that it will have no effect on the flow out of both faucets...??

Apples to oranges - you don't have a positive-displacement water source. The oil system in the aircraft does. It doesn't matter what the downstream pressure is, at a given RPM the oil pump will always pump out the same volume of oil, whether the output pressure is 20 or 200 psi.

As long as the SOURCE of the fluid allows you to change the volume by adjusting the back pressure (vane pump, centrifugal pump, or municipal water suppy), then yes the volume will change. If you have a pump that is positive displacement, the pressure doesn't matter - it's always going to pump the same volume. At higher pressures it just has to work harder to do it.

The point of the oil regulator valve is to maintain XX psi (call it 80 for the sake of argument) and all the oil is directed to the oil galleries until that pressure is achieved - after which time just enough is directed to the oil galleries to maintain that pressure, and the rest is dumped back into the crankcase. The pump puts out quite a bit more oil than is needed, and the regulator makes sure you get everything you need from the pump and discards the rest.

I play with high pressure air compressors (6,000 psi) for a side business, and the oil system on those runs at 850 psi but is designed exactly the same.

[Dave62]

OK lets try this....this is a complicated subject and much misunderstood....and my attempt to make it simple just didn't cut it....
A good visual picture and interactive example of Bernoulli's Principal (yes the same guy that says we can fly) can be viewed at http://home.earthlink.net/~mmc1919/venturi.html
This describes fluid flow and pressure in a system .....note what happens to flow.
Dave

[rocketbob]

Since you guys want to go off in the weeds, and not provide an answer to my question, let me ask you another one.

I fly a Saratoga which has a combination manifold pressure/fuel flow gauge. I overhauled the engine in this airplane, and maintain it, so I am intimately familiar with its workings.

The fuel flow gauge is connected to the flow divider at the top of the engine. The gauge is nothing more than pressure gauge with markings in gallons per hour.

How is it then, that a pressure gauge can be used to indicate volummetric flow?

As the pressure goes up and down on the metered fuel side, more or less fuel is being injected to the intake ports.

There's no need to understand Bernoulli, fluid dynamics, vane-type pumps, positive-displacement pumps, yada yada yada.

The plumbing in my house works the same, and so does the oil system an engine.

Lets compare this to basic Ohms law. Fluid pressure is analgous to voltage. Fluid flow is analagous to current. V=I*R. I=V/R. So as voltage (pressure (V)) decreases, and R stays constant, I (current(flow)) will become less.

[airguy]

*shrugs shoulders, gives up, grabs a beer*

[skylor]

Originally Posted by Bubblehead

Setting the relief valve higher will not cause oil pressure to be higher with warm oil like BruceMe is experiencing. Any relief valve set point higher than the total pressure loss through the engine at operating temperature will not cause more oil to flow through the engine because the relief valve will be shut. The only operating region where the higher relief valve setting helps is when the oil is very cold.

Bubblehead's statement assumes that the oil pressure relief valve is fully closed when the engine oil is at normal operating temperature, but if the pressure relief valve actually behaves as a regulator then this statement is not true. If the relief valve is typically partially open when the engine oil is at normal operating temperature, then increasing its setting will increase the engine oil pressure at normal operating temperatures by bypassing less oil back to the sump. In this case, the Bill Marvel (a two time RV builder, by the way) article that Bob linked in his earlier post has some validity.

Skylor

[Bubblehead]

Skylor - I agree, although I think I did cover the partially open relif valve case. In the operating condition where the relief valve is partially open, changing the relief valve setpoint will change the amount of oil going to the sump, and the remainder will go to the engine.

Bob - I answered your question with a technical argument and references. You answered with an insult. Despite that I will try one more time.

Your bathtub system has nothing to do with the oil system in your airplane. Your fuel flow system has nothing to do with it. They are inherently different than the oil system.

Either do your homework and come back with a logical, technical argument based on a positive displacement pump or drop it.

Oh, on your electrical analogy, we can do it that way, but the electrical equivalent is a constant current source instead of a constant voltage source like a battery. The constant current source will push 7 amps through the system (like the oil pump pushes 7 gpm through the oil system) regardless of the resistance in the system. If the circuit is one simple loop with source and resistor (equivalent to the relief valve being closed) then all 7 amps go throught the resistor (engine). If you add a second loop with a resistor (relief valve open), some current will go through that loop and the rest of the 7 amps will go through the first loop (engine). The relative resistances will determine the balance of current in each loop but the total current will still be 7 amps (or 7 gpm). The constant current source will raise voltage as high as it must to achieve 7 amps of current flow, just like the positive displacement pump will raise pressure as high as it must to achieve 7 gpm.

Constant current source = positive displacement pump system
Constant voltage source = storage battery = your home water system

Here's a diagram. I hope it helps.

[rocketbob]

Originally Posted by Bubblehead

Bob - I answered your question with a technical argument and references. You answered with an insult. Despite that I will try one more time.

Your bathtub system has nothing to do with the oil system in your airplane. Your fuel flow system has nothing to do with it. They are inherently different than the oil system.

Either do your homework and come back with a logical, technical argument based on a positive displacement pump or drop it.

First off I never insulted you. I don't need to insult anyone to make a point.

Secondly, explain how the plumbing analogies are "inherently different". In my house, I have a pump, relief valve, and plumbing. In the engine oil system, there is also a pump, a relief valve, and plumbing (galleys). How's that "inherently different"? Assuming all faucets have a fixed opening for comparison purposes.

Now please answer this question directly, for argument's sake: how is it that a pressure gauge is used on the Saratoga I fly can tell me what the fuel flow is?

Regarding part 2 of your drawing, what happens when R1 is a low value and R2 is a high value? In a current divider circuit the current is not constant at R1 and R2 if they are of different values. The total current stays constant, but at R1 and R2 the current is split proportionally. So if R2 has a high value of resistance, R1 is low, little current will flow thru R2. Therefore I can measure the voltage across R2 and calculate current at R2. And since voltage equates to pressure, and R2 is analagous to the total resistance after the relief valve in a Lyc., the current there is analagous the flow of oil. Nobody cares about the total current since we already know the pump will pump X gpm thru it. Hence. Voltage here can be used to measure current. Hence. Pressure can be used to determine flow.

Now if a third party can please validate the above paragraph, and prove it is incorrect, I will gladly buy John a case of beer.

And I'll add one more point. If R1 (the relief valve) varies in resistance, which it certainly does, the current at R2 (the engine) will vary proportionally to the current at R1. Therefore the flow of oil thru the engine will vary with the position of the relief valve. The total current of the whole circuit will not vary, but again, nobody cares how many GPM the oil pump is flowing, which includes what's dumped into the sump at the relief valve bypass. All we care about is the oil flowing thru the engine (R2) and measuring the voltage across R2 (pressure) will give us a direct indication of the flow of oil thru the engine, relative to the total flow at X rpm by the oil pump.